Battery Backup For A PIC

Hi Guys

I am working on a project that requires a battery backup to keep the pic running, to run a clock.

The PIC runs on 5v, my power supply supplies both 12v and 9v, the 9v feed is supplied to a regulator to supply 5v to my pic.
The 12v feed from the PSU supplies various devices.

The idea is that when the main power shuts down the 12v feed will cut off, stopping all external devices, the 9v feed will be backed up by a 9v PP3 rechargeable battery.

Just wanted to ask if a PP3 battery is acceptable in such an application, i have seen various onboard backup batteries but they are all 3.6v.

Is there a way to make a 3.6v battery work for this application??

cobra1 said:

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Is there a way to make a 3.6v battery work for this application??

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Sure, during the power down period, let the VDD of the PIC sag down to ~3V. That will retain the constants/time keeping...

i didnt think that the PIC could run at such a low voltage..

The datasheet says 4.5v - 5.5v.

The clock is part of the software, when the pic detects that it has lost power (using an I/O port) It will enter a loop where only the clock is running, crystal frequency is 20mhz.

If i just drop the voltage from 5v to 3.6v will it still keep accurate time??

Some PICs will run down to 1.8V, often with slower clock requirements. Which PIC?

pic is a 18f4685

3v0 has suggested using this pic but the LF version which does use a lower voltage, but it doesnt come in the LQFP package tha i wanted to use.

Also at 3.6v what would be an appropriate frequency to run at??

The 18F series have a low voltage detector in hardware might come in handy. When you're running on battery 20MHz will draw a fair amount compared to sleep mode using a 32768Hz crystal on Timer1. Excellent for a RTCC.

If you use an RTC chip it will run off a CR2032 battery for ten years.

Boncuk

Boncuk said:

If you use an RTC chip it will run off a CR2032 battery for ten years.

Boncuk

Click to expand...

I agree.This is the most professional way to do.